HTT and SMP question

Tom tom at
Tue Apr 20 10:24:29 PDT 2004

On Tue, 20 Apr 2004, Kris Kennaway wrote:

> My parallel package builds are slower on build machines with HTT.
> It's not a magic bullet.

  It is more complicated than that.  There isn't just one kind of HTT.
Intel has been tweaking the HTT feature since it was first released.
First generation HTT was known to have some performance irregularities.

  HTT is designed to utilize more instruction units on the CPU at the same
time by running two threads of execution.  So at worst, it shouldn't cause
any slow down versus non-HTT.  However, first generation HTT definitely
did slow down on some work loads.  Intel says they have fixed this in the
newer generation processors.

  There are also FreeBSD factors:  FreeBSD 4 compiled with SMP, avoids a
lot of locking code.  FreeBSD 4 with SMP has a lot of extra locking code,
and that code has cost associated with it.  The cost of SMP locking in
FreeBSD 4, may out way the benefit of HTT.  With FreeBSD 5, the SMP
locking is done quite differently, and it is done all the time anyhow
(since FreeBSD 5 is SMP by default).  So HTT on FreeBSD 5 is more likely
to be beneficial than HTT on FreeBSD 4 (with a non SMP kernel).

> Kris


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